Seems it's OK for some members, but for the rest it's like the picture in first post.I'm sure the solution is easy, I tried already some CSS codes, but they don't work.It's weird that I never use any codes for them signatures.

That's normal. Depending on the profile height and message height, one may be taller than the other. In your screen shot your profile takes up more height, so the message area is left with a blank space on the bottom, because there's not enough content to fill it.